With the recent trends of emission gas regulations from the standpoint of preservation of global environment, reduction of body weight of automobiles has become a critical issue. Thus, a study is being made on reduction of body weight by increasing the strength of the steel sheets used for automobile body and decreasing the steel sheet thickness thereby.
The structural components of automobiles to which such high-strength steel sheets are applied are usually made by press-working and hole-expanding. Thus, the steel sheets, which are the raw material, must have high hole expandability in addition to press workability.
In addition to the issue of environmental preservation, recently, safety of the automobile body is considered as important for protection of passengers in the event of collision. Thus, improvements of impact resistance, which is a measure for safety in the event of collision, are required. For the improvement of the impact resistance, at least the strength of the components of an entire car is preferably as high as possible.
However, in general, increasing the strength of steel sheets decreases the elongation and thus degrades press workability. Moreover, since yield strength is also increased by the increase in strength, there is also a problem of poor shape fixability after pressing. Furthermore, with regard to a high-strength steel sheet mainly composed of a martensitic structure, increasing the elongation to enhance the press workability decreases the hole expandability, and, conversely, increasing the hole expandability decreases the elongation. As such, it is difficult for a steel sheet to achieve both press workability and hole expandability by simply increasing the strength of the steel sheet.
Japanese Unexamined Patent Application Publication No. 2003-221623 discloses an example of an attempt to achieve both press workability and impact resistance, i.e., a cold-rolled steel sheet containing C: 0.02 to 0.15% (on a mass basis, hereinafter the same), Mn: 2.0 to 4.0%, Nb: 0.01 to 0.1%, and the balance being Fe and inevitable impurities, in which the structure is a dual-phase structure (ferrite and second phase) having an average grain size of 5 μm or less. However, according to this technique, not only hot-rolling but also cold-rolling and annealing must be conducted under adequate control to yield a desired structure. Thus, the production cost is high, and load of facilities notably increases if thick steel sheets (4 mm or more) are to be produced. Furthermore, this technique cannot fundamentally overcome the problem of shape fixability.
Also, since continuous annealing and continuous hot dip zincing are concerned, the steel ultimately undergoes heat treatment at 400° C. or more. As a result, it is considered that sufficient strain aging (described in detail below) cannot be obtained due to precipitation of stable iron carbide (cementite) and a decrease in amount of solute carbon.
As described above, a hot-rolled steel sheet that has low strength, high press workability, and high hole expandability during forming of automobile components and exhibits high strength and high impact resistance when the sheet is worked into a finished product has been strongly demanded.
As a related art handling such a demand, a bake-hardenable steel sheet has been developed under an aim of obtaining a steel sheet that has high strength and further, high press workability. This steel sheet features an increased yield stress by subjecting it to a bake-finish process (including retaining at a constant temperature of 100° C. to 200° C.) after press working.
This steel sheet has a structure in which ferrite is the matrix and the amount of the solute carbon in a solid-solution state is controlled in an adequate range. This steel sheet is soft during press working and dislocations are introduced into the ferrite during forming. During the bake finishing conducted after the press working, the solute carbon remaining therein is hooked to dislocations to pin the dislocations, thereby increasing the yield stress. In the past, a phenomenon of an increase in yield strength has been traditionally referred to as strain aging.
However, although the yield stress can be increased by the bake-hardenable steel sheet, the tensile strength cannot be increased. The effect is also not sufficient with regard to impact resistance.
Japanese Unexamined Patent Application Publication No. 62-74051 discloses a hot-rolled high-tensile strength steel sheet having excellent strain aging property and aging resistance (resistance to deterioration of material properties due to room-temperature aging, aging resistance at RT), the sheet containing C: 0.08 to 0.2%, Mn: 1.5 to 3.5%, and the balance being Fe and inevitable impurities, the structure of the sheet being a multi-phase structure containing 5% or less of ferrite, and bainite or partially containing martensite.
Although the hot-rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 62-74051 has high strain aging property, the tensile strength still cannot be increased. The effect on improvement of impact resistance is insufficient.
Japanese Unexamined Patent Application Publication No. 4-74824 discloses a hot-rolled high-tensile strength steel sheet having excellent strain aging property and aging resistance, the sheet containing C: 0.02 to 0.13%, Si: 2% or less, Mn: 0.6 to 2.5%, and the balance being Fe and inevitable impurities and having a dual-phase microstructure mainly composed of ferrite and martensite.
Despite the strain aging property of the hot-rolled steel sheet described in Japanese Unexamined Patent Application Publication No. 4-74824, the tensile strength is still not improved, and the effect on the improvement of the impact resistance is insufficient. There is also a drawback of poor hole expandability.
Japanese Unexamined Patent Application Publication No. 10-310824 proposes a method for making a galvannealed steel sheet that uses a hot-rolled steel sheet or a cold-rolled steel sheet as the black plate, in which the strength is expected to increase by heat treatment after working. This is the technology in which a steel containing C: 0.01 to 0.08%, adequate amounts of Si, Mn, P, S, Al, and N, and 0.05 to 3.0% of at least one of Cr, W, and Mo in total is hot-rolled (and additionally cold-rolled and optionally temper-rolled and annealed), and subjected to galvanizing and then to thermal alloying. The resulting steel sheet has a microstructure of a ferritic single phase, ferrite+pearlite, or ferrite+bainite.
Japanese Unexamined Patent Application Publication No. 10-310824 teaches that the tensile strength can be increased by heating the resulting steel sheet in the temperature range of 200° C. to 450° C. after working. However, high ductility and low yield strength are not achieved, and there is a problem of decreased press workability.
Components of automobile bodies are under repeated stresses and are required to exhibit excellent fatigue property in addition to the above-described properties. In particular, these requirements are more acute when the sheet thickness is reduced by increasing the strength.
As an technique aiming to improve the fatigue property, Japanese Unexamined Patent Application Publication No. 11-199975 proposes a hot-rolled steel sheet for processing working having excellent fatigue property, the sheet containing C: 0.03 to 0.20%, adequate amounts of Si, Mn, P, S, and Al, Cu: 0.2 to 2.0%, and B: 0.0002 to 0.002%, the microstructure being a dual-phase structure including a ferritic dominant phase and a martensitic second phase, in which the state of existence of Cu in the ferrite phase is a solid solution state and/or a precipitation state of 2 nm or less.
However, the steel sheet described in Japanese Unexamined Patent Application Publication No. 11-199975 does not show how all the press workability, hole expandability and the impact resistance are achieved at the same time. Moreover, since addition of Cu is necessary, there is also a problem of difficulty of scrapping and recycling.
As described above, there has been strongly demanded a hot-rolled steel sheet that exhibits a low TS and high press-workability and hole-expandability during forming of the automobile components but high TS and impact resistance once the sheet is worked into a finished product, and a hot-rolled steel sheet having excellent fatigue in addition to these properties. However, the technology that enables stable industrial production of steel sheets that satisfy all of these properties has not been available.
It could therefore be advantageous to provide a hot-rolled steel sheet suitable for automobile steel sheets, the hot-rolled steel sheet having excellent press-workability and hole-expandability and excellent strain aging property by which the tensile strength notably increases after press forming by heat treatment at about the same temperature as that of the known baking process. It could also be advantageous to provide a hot-rolled steel sheet having significantly improved fatigue property in addition to the strain aging property, and to provide a method that enables stable manufacturing of these hot-rolled steel sheets.